Method and apparatus for fusion splicing optical fibers

ABSTRACT

We have discovered that the strength of arc fusion splices in optical fiber can be adversely affected by particles (e.g., SiO 2  particles) from the electrodes. Disclosed is a method of arc fusion splicing that can substantially increase the probability that a given fiber splice will meet a given strength requirement. The method comprises initiating the arc in a &#34;cleaning&#34; position selected such that the probability of incidence on the fibers of particles from the electrodes is relatively low, followed by changing the relative position between the electrodes with the arc therebetween and the fibers to the conventional &#34;heating&#34; position and forming the splice.

This application is a continuation of application Ser. No. 08/056971,filed on May 3, 1993, now abandoned.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for arc fusion splicingoptical fibers.

BACKGROUND OF THE INVENTION

For many optical fiber applications, it is necessary to splice twoseparate lengths of fiber together to form a single spliced length offiber. Frequently, these splices need to have high strength, comparableto the strength of the fiber.

Among known fiber splicing techniques is arc fusion splicing, which canbe an automated process. Exemplarily, a computer controlled fiberpositioner aligns the fiber ends face-to-face until optimumfiber-to-fiber optical transmission is achieved. A current is thensupplied to two electrodes, with the resulting electric arc heating theoptical fibers such that the two abutting fiber ends are fused together.See, for instance, D. L. Bisbee, "Splicing Silica Fibers With anElectric Arc", Applied Optics, Vol. 15, No. 3, Mar. 1976, pp. 796-798.

It is known that optical fiber splices that are formed by means of arcfusion frequently have relatively low strength, frequently less thanhalf of the intrinsic strength of the glass. This is a drawback thatlimits the usefulness of this, otherwise advantageous, splicingtechnique. Thus, it would be desirable to have available an arc fusionsplicing method and apparatus that can produce a higher percentage ofhigh strength splices than is typically obtained with prior arttechniques and apparatus. This application discloses such a method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary SEM (scanning electron microscope) micrographof an electrode tip;

FIG. 2 shows a SEM micrograph of a portion of a fiber fracture surfacewith associated particle;

FIGS. 3 and 4 present exemplary strength data for fibers splicedaccording to the invention and according to the prior art; and

FIG. 5 schematically depicts exemplary apparatus according to theinvention.

THE INVENTION

This invention is embodied in a method and apparatus for splicing theends of two optical fibers to form a single length of optical fiber, andcan yield a higher percentage of splices of strength above a givenminimum strength than is typically attained with prior art arc fusionmethods and apparatus.

We have made the surprising discovery that the, frequently observed,relatively high percentage of low strength arc fusion splices istypically associated with particulate emission from the electrodes,especially during the initial period of arc operation. FIG. 1 is a SEMmicrograph of the tip of an electrode and shows a plethora of particles,typically SiO₂ particles, believed to have been deposited duringprevious arc fusion splicing operations. Incidence of a particle on oneof the optical fibers being spliced can result in a relatively lowstrength splice, since the particle can act as stress concentrator. FIG.2 is a SEM micrograph of a fracture surface of an arc fusion splicedsilica fiber, and shows a silica particle associated with the fracture.It is thus an object of the invention to provide an arc fusion method(and apparatus for the practice of the method) that at leastsignificantly reduces the probability that a particle from an electrodeis incident on the optical fiber.

More specifically, the inventive method of end-to-end fusion splicingtwo lengths of optical fiber typically comprises positioning an end ofeach of the lengths of fiber such that the ends are substantiallyaligned in linear opposing fashion, and fusing the fiber ends by aprocess that comprises heating the ends by means of an electric arcbetween electrodes, with the electrodes in a first ("heating") positionrelative to the fiber ends. Significantly, the method also comprisesstarting the arc with the electrodes in a second ("cleaning") positionrelative to the fiber ends, and maintaining the electrodes with the arctherebetween in the cleaning position for at least a time t_(c),followed by placing the electrodes with the arc therebetween and thefiber ends in said heating position. Associated with the electrodes withthe arc therebetween is a rate of particle emission, and t_(c) is thetime at which the rate of particle emission has dropped to 50% of theinitial rate of particle emission at the time the arc is started.Preferably the change from the cleaning to the heating position is notmade until the particle emission rate has dropped below 90% of theinitial rate. This time can be as short as a fraction of a second,possibly a few seconds, since the rate of particle emission typically ishigh when the arc is struck, but drops rapidly thereafter.

The invention is also embodied in fusion splicing apparatus thatcomprises means for holding an end portion of each of two lengths ofoptical fibers, means for aligning the ends, means for maintaining anelectric arc between (two or possibly more) electrodes, and means thatfacilitate placing said fiber ends and said electrodes into a first anda second position relative to each other, with the first ("heating")position selected such that the arc can heat said fiber ends, and thesecond ("cleaning") position preferably selected such that substantiallyno particles emitted from the electrodes with the arc therebetween canreach the fiber ends.

The effectiveness of the method according to the invention isillustrated by the exemplary data of FIGS. 3 and 4, wherein numerals 30and 40 refer to prior art results, and 31 and 41 to fibers splicedaccording to the invention. The data of curve 31 were obtained fromconventional optical fibers that were arc fusion spliced according toone embodiment of the invention, namely, by a method that involvesmaintaining the fiber ends in close proximity to each other, and movingthe electrodes with the arc therebetween from the cleaning position intothe heating position. On the other hand, the data of FIG. 4 wereobtained from such fibers that were arc fusion spliced according toanother embodiment of the invention, namely, by a method that involvesplacing the fiber ends into a facing position, with the ends separatedfrom each other by a significant distance, starting the arc, and movingthe fiber ends from the cleaning (remote from each other) to the heating(adjacent to each other) position without movement of the electrodes.

As FIGS. 3 and 4 show, both embodiments of the invention yieldsubstantial improvement in splice strength, e.g., from median strengthsof 359 and 305 ksi (2.48 and 2.10 GPa, respectively) to 489 and 530 ksi(3.37 and 3.66 GPa, respectively), respectively. Clearly, in both casesa substantially higher percentage of splices made according to theinvention passes at a given test level (e.g., at 400 ksi, i.e., 2.76GPa) than do fibers spliced according to the prior art. Furthermore, thevalues for splices made according to the invention are less widelyscattered (as expressed by the coefficient of variation v) than thosemade by the prior art technique. Differences in median value between thecorresponding data of FIGS. 3 and 4 are currently not considered to besignificant.

FIG. 5 schematically depicts an exemplary embodiment of apparatusaccording to the invention. Fusion splicer 50 comprises fiber holdingmeans 520 and 521 (e.g., known vacuum chucks), fiber aligning means 530and 531 (e.g., comprising known servo-controlled micropositioningmeans), and means for maintaining an arc (comprising an appropriatepower supply 54) between electrodes 550 and 551. FIG. 5 also showsoptical fibers 510 and 511, control unit 54, and arc 56. Exemplarily,the Z-direction is parallel the fiber axis, the X-Z plane is the"horizontal" plane, and the Y direction is normal to the X-Z plane,positioning means 530 can adjust the position of the fiber 510 in the Yand Z directions, and positioning means 531 can adjust the position offiber 511 in the X and Z directions. As seen in FIG. 5, fibers 510 and511 are positioned outside arc 56 during the cleaning process.

Those skilled in the an will appreciate that FIG. 5 is a schematicillustration of functional elements, and that various (necessary butconventional) parts are not shown. For instance, all the parts shown inFIG. 5 will typically be integrated into a single unit, requiringprovision of mounting means and housing means. Indeed, apparatusaccording to the invention is likely to resemble prior art apparatus(e.g., Ericsson 905 Fusion Splicer), but comprising the additionalfeatures (possibly embodied in software) that enable it to carry out theinventive method.

On a modified 905 Fusion Splicer, the method was practiced as follows.After conventional preparatory steps such as coating stripping, fibercleaning and cleaving, the fibers were mounted in the apparatus, andpositioned such that the ends almost touched, and the fibers wereoptically aligned in the X and Y directions. Next, each fiber was movedabout 2 mm in the Z-direction, away from the electrodes, resulting in a4 mm separation between the fiber ends. This is the "cleaning" positionin this exemplary embodiment. After attainment of the cleaning position,the arc was initiated (e.g., 6 mA current) and maintained. A few moments(e.g., 3 seconds) after are initiation the fibers were moved in theZ-direction until the ends are just touching. This is the conventionalheating position. The current was increased to 10.5 mA, the fiberZ-direction movement continued (about 15 μm), current was increased (toabout 16.3 mA), such that fusion occurred, with continued fiberZ-direction movement (about 6 μm). This completed the process. Currentwas shut off, and the optical loss of the splice measured. This wasfollowed by conventional steps such as annealing and re-coating of thesplice region.

Clearly, the geometrical relationship between electrodes and fibers inthe cleaning and heating positions, respectively, is a matter of designchoice, and those skilled in the art will be able to devise manydifferent embodiments other than the above described one. All suchembodiments are contemplated, provided only that they meet therequirement that the probability of particle incidence on the fiber fromthe electrodes is substantially reduced (e.g., by at least 50%,preferably at least 90%). We would like to emphasize that the rate ofparticle emission typically is highest at the moment of are initiation,dropping typically rapidly thereafter. Rate of particle emission can, atleast in principle, be readily determined, e.g., by means of adeposition rate monitor.

Among contemplated embodiments is apparatus that comprises means formoving the electrodes (typically tungsten) to the cleaning position andthe heating position. This is readily implemented by connecting theelectrode holders to (servo-controlled or manual) positioning means.Exemplarily, the electrodes are moved 5 mm or more in the Y-direction.Hybrid embodiments (e.g., embodiments comprising movement of theelectrodes as well as of the fibers) are also contemplated.

Also among contemplated embodiments is apparatus that provides aphysical barrier between the electrodes and the fiber at arc initiationand for a few moments thereafter, with no movement of either theelectrodes or the fibers. Such a barrier could be a silica sleeve orsleeves.

We claim:
 1. A method for fusion splicing two lengths of optical fibercomprising:providing at least two electrodes having an electricarc-forming space therebetween, said electrodes having associatedtherewith a rate of particle emission including an initial rate ofparticle emission at the time the arc is started, positioning an end ofeach of two lengths of optical fiber such that the fiber ends aresubstantially aligned in a linearly opposed manner and such that thefiber ends are positioned outside the electric arc-forming space;initiating an electric arc between said at least two electrodes for atleast a time t_(c), where t_(c) is the time at which the rate ofparticle emission is 50% of the initial rate of particle emission;positioning the fiber ends within the electric arc-forming space; fusingthe fiber ends by heating with an electric arc formed within theelectric arc-forming space.
 2. A method according to claim 1 wherein thefiber ends are moved and the at least two electrodes are not movedbetween the position outside the electric arc-forming space and theposition within the electric arc forming space.
 3. A method according toclaim 1 wherein the at least two electrodes are moved and the fiber endsare not moved between the position outside the electric arc-formingspace and the position within the electric arc forming space.
 4. Amethod according to claim 1 wherein the fiber ends and the at least twoelectrodes are moved between the position outside the electricarc-forming space and the position within the electric arc formingspace.
 5. A method according to claim 1 wherein each of the two lengthsof optical fiber includes a core and said step of positioning the fiberends includes aligning the cores.
 6. A method according to claim 1wherein the positioned outside the electric arc-forming space comprisesplacing a physical barrier between the at least two electrodes and thefiber ends.